Various measures for reduction of emissions of CO2 are being taken in order to deal with atmospheric pollution and global warming. In particular, in the automobile industry, the reduction of emissions of CO2 is highly expected in association with the spread of electric vehicles and hybrid electric vehicles. Thus, development of high-performance secondary batteries serving as driving power sources of motors for use in such vehicles, is actively being carried out. For the secondary batteries serving as driving power sources of motors, a higher capacity and cycle property are particularly required. Thus, lithium ion secondary batteries having high theoretical energy are gaining increased attention among other types of secondary batteries.
The lithium ion secondary batteries are required to store a large amount of electricity per unit mass in positive electrodes and negative electrodes in order to increase energy density of the lithium ion secondary batteries. Therefore, the determination of active materials used in the respective electrodes is quite important so as to fulfill such a requirement.
There is known a method for manufacturing an electrode material used for a lithium ion secondary battery having a high discharge capacity per volume and a high charge-discharge cycle property, as proposed in Patent Literature 1. In particular, Si fine particles having a predetermined average particle diameter and specific surface area are prepared in a manner such that powder containing Si as a main component is pulverized with a wet medium mill. Metal powder containing a predetermined element such as Sn or Al and carbon powder are then added to the Si fine particles, followed by dry milling with a ball mill. Thus, an electrode material including composite particles having a predetermined average particle diameter and specific surface area is manufactured according to the method described above. Patent Literature 1 further teaches that the electrode thus obtained is used as a negative electrode for a lithium ion secondary battery.